1. FIELD OF THE INVENTION
The present invention relates to a gain control circuit which can be suitably used in single side-band receiver, tape recorder or the like apparatus. More particularly, the invention relates to a gain control circuit for audio or voice frequency amplifier circuits or apparatus.
2. DESCRIPTION OF THE PRIOR ART
In the gain control circuit for the audio frequency amplifier circuit destined to amplify the audio or voice signal which has usually varying signal levels in a large range of magnitude, it is generally accepted that the gain control element of the control circuit should preferably have a short control time, while the restoring time of the control element is to be long. In this connection, the term "control time or duration" or "control operation or duration" as herein used means the time interval which is required for starting the operation of the gain control circuit from response to an audio input signal thereto. Further, the term "restoring time or duration" or "restoration time or duration" means the time span which elapses until the gain control potential has resumed the original or starting level, as measured from the time point at which the input signal disappeared. When the control time is short, the gain control circuit can rapidly respond to the input signal, whereby the input signal can be advantageously controlled without distortion. On the other hand, the restoring time of a long duration provides such advantage that the input signal having a relatively high signal level which follows the disappearance of a preceding input signal of the substantially same signal level may undergo an adequate gain control operation.
The hitherto known gain control circuit of the above described type has certainly a short control time. However, the restoring time of the conventional gain control circuit is disadvantageously varied in dependence on the magnitude of the input signal level. In other words, when the level of the input signal is low, the restoring time becomes short and vice versa regardless of the input duration, i.e. whether the input signal is of a single shot variety or continuous type. Thus, the restoring time becomes longer for the high level input signal, even when the input signal is of the single shot kind such as a spike pulse. This feature of the hitherto known gain control circuit brings about certain disadvantages. For example, assuming that a low level audio signal is supplied to the gain control circuit after the application of an audio-frequency signal of the one shot nature having a relatively high signal level, then the succeeding low level signal will be attenuated, because the gain of the input amplifier circuit is rapidly lowered by the preceding high level signal.
Due to the above described characteristic, the hitherto known gain control circuit will provide serious disadvantage in certain applications thereof. For example, it is assumed that the conventional gain control circuit is employed in a transmitter-receiver apparatus. As is well known, a switching noise signal of a relatively high level will be produced, when such apparatus is switched into the receiver mode from the transmitter mode. The gain control circuit will then operate in response to the input noise signal with the aforementioned characteristic of the control and the restoring time durations. In such situation, a leading portion of the audio or speech signal received by the apparatus operating now in the receiver mode may be undesirably attenuated or cut off, since the gain of the input amplifier circuit destined to amplify the input signal would be still in a controlled level which has been involved by the preceding switching noise due to the long restoring time of the gain control circuit.